The invention relates to heat treatment of superalloys. More particularly, the invention relates to stabilization of nickel-based alloys for disks and other gas turbine engine rotating parts.
The combustion, turbine, and exhaust sections of gas turbine engines are subject to extreme heating as are latter portions of the compressor section. This heating imposes substantial material constraints on components of these sections. One area of particular importance involves structural rotating parts such as blade-bearing turbine disks and shafts. The disks are subject to extreme mechanical stresses, in addition to the thermal stresses, for significant periods of time during engine operation. Shafts are subject to somewhat similar stresses and variant alloys have been developed for shaft use.
Exotic materials have been developed to address the demands of turbine disk use. Shafts are subject to somewhat similar stresses and variant alloys have been developed for shaft use. Separately, other materials have been proposed to address the demands of turbine blade use. Turbine section blades are typically cast and some blades include complex internal features.
U.S. Pat. Nos. 5,120,373 and 5,938,863 disclose advanced nickel-base superalloys. One commercial disk alloy embodiment of such an alloy has a nominal composition of 16.0Cr, 13.5Co, 4.15Mo, 4.6Ti, 2.2Al, 0.07Zr, 0.006B, 0.0025Mg, balance Ni, by weight percent. For reference, this alloy is identified as alloy “A” hereafter. A commercial shaft alloy variant has a nominal composition of 15.75Cr, 13.5Co, 4.15Mo, 4.6Ti, 2.2Al, 0.07Zr, 0.006B, 0.0025Mg, balance Ni, by weight percent. For reference, this alloy is identified as alloy “B” hereafter. Alloy “B” is a higher tensile strength alloy. Both are used in a conventionally processed (not powder metallurgical) form.
U.S. Pat. No. 6,521,175 discloses an advanced nickel-base superalloy for powder metallurgical manufacture of turbine disks. The '175 patent discloses disk alloys optimized for short-time engine cycles, with disk temperatures approaching temperatures of about 1500° F. (816° C.). Other disk alloys are disclosed in U.S. Pat. No. 5,104,614, US2004221927, EP1201777, and EP1195446.
An exemplary processing of a forging includes: solution treatment; stabilization; and age hardening stages. Exemplary solution treatment comprises heating to a high temperature effective to remove prior precipitate phases (principally gamma prime (γ′)). An exemplary temperature is in excess of 1900° F. (e.g., 1910-2015° F. in standard alloy “A” processing with an upper limit reflecting a desired control of grain size). Such a temperature is maintained for an interval effective to achieve desired precipitate phase removal (e.g., two hours in standard (prior art) alloy “A” processing). Air cooling or a faster cooling rate is then performed to rapidly decrease temperature to avoid precipitate formation at undesirable intermediate temperatures. An exemplary cooling is to a temperature near or below 1000° F.
Stabilization serves to form carbides at grain boundaries. Exemplary stabilization comprises heating at an intermediate temperature effective to form sufficient carbides to stabilize the grain boundaries (e.g., 1500+/−25° F. in standard alloy “A” processing). Such a temperature is maintained for an interval effective to achieve the desired carbide formation (e.g., four hours in standard alloy “A” processing). Fan air cooling or an equivalent is then performed to similarly avoid any precipitate formation at undesirable intermediate temperatures. An exemplary cooling is to a temperature near or below 1000° F.
Age hardening (precipitation heat treatment) serves to grow desired γ′ within the γ matrix. Exemplary age hardening comprises heating at a lower temperature and for a time effective to grow a desired size and volume fraction of γ′ (e.g., 1350+/−25° F. for eight hours in standard alloy “A” processing). Air cooling or fan air cooling is then performed to rapidly terminate γ′ formation.